With the realization of ultrashort pulsed laser technology in recent years, there has been remarkable progress in emission technology and detection technology of pulsed coherent electromagnetic waves in the infrared region (0.01 to 130 THz). As a result, time-domain pulsed spectroscopy using these pulsed electromagnetic waves in the infrared region has become possible, and there have been pioneering developments towards realization of time-domain pulsed spectrometer apparatuses.
Time-domain pulsed spectroscopy is a spectroscopic method in which the time-dependent electric-field intensity of a pulsed electromagnetic wave is measured, and by Fourier Transform that time-dependent data (time-domain data), the electric-field intensity and phase of each frequency component constituting that pulse are obtained. One feature of this spectroscopic method is that the measurement wavelength region is at the boundary between light and radio waves, which has been difficult to measure conventionally. Therefore, elucidation of the properties of novel materials and new phenomena using this spectroscopic method has been long anticipated. With conventional spectroscopy, only the electric-field intensity of an electromagnetic wave can be obtained; however, a unique feature possessed by this time-domain pulsed spectroscopy is its ability not only to measure the electric-field intensity (amplitude) of the electromagnetic waves but also to obtain the phase, by directly measuring the temporal variations of the electric-field intensity of the electromagnetic wave. Therefore, it is possible to obtain a phase-shift spectrum by comparing with a case where no sample is present. Because the phase-shift is proportional to the wavenumber vector, it is possible to determine the dispersion relation of the sample using this spectroscopic method, and it is also possible to determine the dielectric constant of a ferroelectric crystal from this dispersion relation (see Patent Document 1).
FIG. 7 shows an example of a conventional time-domain pulsed spectrometer apparatus.
A femtosecond laser is used as a light source 1. For example, a mode-locked erbium (Er)-doped fiber laser is used as the light source 1. This mode-locked fiber laser 1 transmits, for example, a femtosecond laser pulse L1 with an average power of 10 mW, at a wavelength of 780 nm, a pulse width of 120 fs, and a repetition rate of 48.5 MHz.
The femtosecond laser pulse L1 emitted from the light source 1 is split by a beamsplitter 2. The femtosecond laser pulse at one side, serving as excitation pulsed laser light L2, is radiated onto pulsed-light emitting means (infrared light emitting device) 5. At this time, after the excitation pulsed laser light L2 is modulated by an optical chopper 3, it is focused by an objective lens 4. This pulsed-light emitting means 5 is, for example, a photoconductive antenna; when the excitation pulsed laser light L2 is radiated, an instantaneous current flows, and a far-infrared pulse L3 is emitted. This far-infrared pulse (THz (terahertz) light pulse) L3 is guided by parabolic mirrors 6 and 7 and irradiates a measurement sample 8. Reflected or transmitted pulsed light (in the figure, it is transmitted pulsed light) L3′ from the sample 8 is guided to detecting means 12 via parabolic mirrors 9 and 10.
The other laser light split at the beamsplitter 2 is guided to the detecting means 12 to serve as detection pulsed laser light L4. Because this detecting means 12, which is also a photoconductive antenna, is irradiated with the detection pulsed laser light L4 and becomes conductive only momentarily, it is possible to detect the electric-field intensity of the reflected or transmitted pulsed light from the sample 8 arriving at that instant as an electrical current. A time-domain signal of the electric-field intensity of the reflected or transmitted pulsed light from the sample 8 can be obtained by imposing a time delay at predetermined time intervals to the detection pulsed laser light L4 with respect to the excitation pulsed laser light L2 using optical delay means 13 (or 14). In this figure, in addition to the optical delay means 13 (or 14) for time-domain signal measurement, an optical delay means 14 (or 13) for time-origin adjustment may also be provided.
The time-division data of the electric-field intensity of the reflected or transmitted pulsed light from the sample 8 is processed by signal processing means. In other words, the amplitude and phase shift spectra of the electric-field intensity of the reflected or transmitted pulsed electromagnetic wave from the sample 8 is obtained by sending the data to a computer 17 via a lock-in amplifier 16, and then recording it as time-domain data and subjecting a time-domain data set to Fourier transform processing in the computer 17 to convert it to the oscillation frequency (frequency) domain.
FIG. 8 shows the pulsed light emitting means 5. The pulsed light emitting means uses a photoconductive switch element (antenna electrode) with a dipole antenna structure formed on a photoconductive layer made of low-temperature growth gallium arsenide (LT(low temperature)-GaAs). Thus, to generate terahertz emission light L3, the terahertz emission light L3 is obtained by irradiating such a pulsed light emitting means 5 with excitation pulsed laser light L2 to excite free carriers, namely electrons and holes, and performing ultrahigh-speed current modulation. In other words, when the pulsed light emitting means 5, to which a bias current is applied, is irradiated with the excitation pulsed laser light L2, the electric field is oscillated. When the electric field is oscillated, due to the oscillating current, the terahertz emission light L3 having a broad spectral distribution over a frequency range defined by a temporal width Δt of the excitation pulsed laser light L2 irradiating the pulsed-electromagnetic-wave emitting element 5 is obtained.
The detecting means 12 has the same configuration as the pulsed light emitting means 5 shown in FIG. 8. When this detecting means 12 is simultaneously irradiated with the terahertz light L3′ transmitted through the sample and the detection pulsed laser light L4, it is possible to measure the intensity of the terahertz light L3′ transmitted through the sample during the time period when the detection pulsed laser light L4 is irradiated.
FIG. 9 shows antenna electrodes 21 formed on a photoconductive layer 22 made ot LT-GaAs. Gold (Au) is used as the antenna electrodes 21. As shown in the magnified view of an antenna peripheral portion 21c at the right of this figure, a gap of about 5 μm is formed between the pair of antenna electrodes 21, and the width of the antenna electrodes is about 10 μm.
To perform polarization analysis using such a time-domain pulsed spectrometer apparatus, a known technique for obtaining a desired polarization involves inserting a polarizer into the light path (see Patent Document 2).
FIG. 10 shows a sectional view, along A-A′, of the antenna peripheral portion 21c shown in FIG. 9. As is clear from this figure, the photoconductive layer 22 is formed on a substrate 23 which is made of semi-insulated gallium arsenide (SI(semi-insulated)-GaAs).
When used as the pulsed light emitting means 5, the excitation pulsed laser light L2 is irradiated towards the antenna electrode 21 side (one side surface) (from the top in the figure) as viewed from the photoconductive layer 22. The terahertz emission light L3 is emitted towards the substrate 23 side (other side surface) (towards the bottom in the figure) as viewed from the photoconductive layer 22.
When used as the detecting means 12, the detection pulsed laser light L4 is irradiated towards the antenna electrode 21 side (one side) (from the top in the figure) as viewed from the photoconductive layer 22. The terahertz light L3′ transmitted through the sample is irradiated from the substrate 23 side (other side) (from the bottom in the figure) as viewed from the photoconductive layer 22.
Patent Document 1:
Japanese Unexamined Patent Application, Publication No. 2002-277394.
Patent Document 2:
Japanese Unexamined Patent Application, Publication No. 2003-014620.